The present invention generally relates to the field of electrochemistry, and more particularly, to an electrochemical cell having a metal hydride electrode.
Metal hydride systems show a great deal of promise as electrical energy storage devices due to their high theoretical energy and power densities, rechargeability, and potential broad range applications. Although the first intermetallic hydride was developed thirty years ago and despite construction of the first prototype metal hydrogen electrode twenty years ago, metal hydrogen electrodes still experience many shortcomings. These include high cost of alloys, poor hydrogen storage capabilities, difficult activation, pyrophoricity, problems of impurities, thermodynamic instabilities, embrittlement, and corrosion in alkaline media.
There are two classes of metal hydride alloys employed as negative electrodes. They are the AB.sub.2 and AB.sub.5 (LaNi.sub.5) alloys, where A may be represent magnesium, zirconium, and lanthanum, and B may be substituted by nickel, vanadium, chromium, or manganese. Both classes of alloys include many additional metal components which improve performance, life, and self-discharge. The rate at which AB.sub.2 or AB.sub.5 alloys can absorb hydrogen generated electrochemically and the degree of loading is greatly influenced by the interphase region formed when an electrode is exposed to an electrolyte. In the simplest case, the interphase region takes the form of the electrical double layer. In the more complex cases and, in particular, during the charge transfer reaction, it consists of layers, each associated with a participating elementary process. In this representation, the interphase region is an open system in which a number of consecutive processes takes place, of which the slowest one determines the rate. These processes include transport of the reactants from the bulk to the electrode surface by diffusion, adsorption on the electrode surface, charge transfer, desorption of the reaction products, followed by transport of the reaction products away from the electrode surface. In a discharging battery, these same processes occur; however, in a battery the electrons ultimately flow into an external circuit where the electrical work is delivered. In the negative electrode, the relevant processes during charge/discharge of a metal hydride battery occurs in a multi phase environment--gas, liquid, and solid.
The central role of the interphase in transport of electrochemically generated hydrogen into the electrode interior has been discussed recently and the non-autonomous character of the interphase was stressed. It was shown that the interphase is an active element and that its properties are determined by those of the contacting phases, i.e., the electrode interior as well as the electrolyte. The interphase region may be affected in the course of battery operation, especially as a result of cycling. During charging/discharging operational modes, the electrode matrix expands and contracts. With cycling, this mechanical stress results in embrittlement and consequent loss of performance by the electrochemical cell.
Therefore, a need exists for an additive which would 1) control both electrodic reactions and transport properties; 2) allow high degrees of H/M loading, and 3) reduce the effects of mechanical stress during cycling to provide increased life of batteries and/or fuel cells.